麻豆传媒

AS MUCH AS we may want to, we cannot go back in time. Is this just our current reality, or is the direction of time fixed by the laws of the universe?

One clue to explain this 鈥渁rrow鈥� of time is the second law of thermodynamics. This states that entropy 鈥� a measure of disorder in a system, loosely speaking 鈥� always tends to increase. Unless you put work into a system to maintain order, like tidying your bedroom, things will get messier.

While most physicists agree that there is a link between entropy and the arrow of time, how they relate is disputed. Some physicists think increasing entropy gives time its arrow, while some say the arrow is just an illusion. Others think we lack a basic understanding of time and, perhaps, marrying the quantum and classical worlds together will lead us to a new way of thinking about it. Some theories do away with entropy in the picture of time altogether.

While on the macroscale, entropy and the laws of thermodynamics go some way to explaining why things go in one direction, the arrow of time was a puzzle on the small scale for many years. In fact, microscopic interactions can and do occur both forwards and in reverse. To us, this may seem strange. 鈥淥ur intuition about time tends to be very bad,鈥� says at the University of California, Berkeley, 鈥渂ecause we鈥檙e sort of embedded in it.鈥�

But Crooks has shown that entropy plays a role on the small scale, too. Take any chemical reaction: its efficiency is always limited by microscopic rebellions, where the reverse reaction starts to take place. Crooks has developed a series of theorems that quantify the difference in the probability for reactions going forwards or backwards, based on the difference in entropy produced (see 鈥�Quantum motors鈥�), showing that there is an arrow of time on the micro scale.

Could we ever travel back in time? The absence of hordes of tourists from the future suggests not. However, some solutions to the equations of general relativity appear to provide a route to the past.

In Einstein鈥檚 theory, the acceleration towards massive objects caused by a gravitational field is described as a curvature in space-time itself. Curve something enough, with enough gravity, and you could form a loop. These closed time-like curvatures (CTCs) crop up in various theoretical descriptions of the universe, from swirling dust to spinning cosmic strings, which are one-dimensional blips in space-time that could span astronomical distances.

鈥淭he arrow of time leads forward, as measured locally by the observer, but globally he or she may return to an event in the past,鈥� says at the University of Lisbon in Portugal. This could open the door to all manner of mischief, such as the paradox of killing your own grandfather before your father was born. Fortunately, most scenarios that lead to CTCs are deemed impossible to physically occur.

鈥淚t is extremely unlikely that CTCs exist naturally in our universe,鈥� says Lobo. However, he leaves open the possibility that if an infinitely advanced civilisation could construct, for instance, a traversable wormhole, a tunnel between two points in space and time, then who knows? Maybe they could create a CTC and travel backwards in time after all.